This invention relates to olefin polymerization catalysts, and more particularly, to catalysts which comprise (a) the reaction product of an organoaluminum compound with a mixture comprising a titanium compound, a zirconium compound, and a vanadium compound; and (b) an organoaluminum promoter, and which are useful in the preparation of olefin polymers, and particularly ethylene polymers, having a broad and controllable molecular weight distribution.
In the low pressure polymerization of olefins, it is well known to employ high activity catalysts comprising an organoaluminum promoter and a component prepared by reducing one or more transition metal compounds with an organoaluminum compound. The use of such high activity catalysts is particularly advantageous in that polyolefins can be produced in yields which are sufficiently high to reduce the relative concentration of catalyst residues in the polymeric product to such an extent that a commercially useful product can be obtained without the need for removal of catalyst residues. However, the processability of the polymeric product and the suitability thereof for use in a variety of applications also are important factors, and to these ends, it is desirable to provide polyolefins having a broad molecular weight distribution and to be able to control molecular weight distributions. As is well known, polyolefin molecular weight distributions often vary depending upon the choice of polymerization catalyst, and, for a given catalyst system, variations in preparation of the catalysts or components thereof often affords means for exercising some degree of control over molecular weight distributions.
Prior art proposals to provide high activity catalysts containing combinations of transition metal compounds and useful in the preparation of polyolefins having a broad and controllable molecular weight distribution have met with varying degrees of success. One such proposal can be found in U.S. Pat. No. 3,218,266, which discloses olefin polymerization catalysts comprising an alkylaluminum promoter and a component prepared by treating a mixture of a titanium (IV) alkoxyhalide and vanadium tetrachloride or vanadium oxytrichloride with a reducing agent such as an alkylaluminum compound. U.S. Pat. No. 3,859,267 discloses olefin polymerization catalysts comprising an alkylaluminum promoter and a component prepared by reducing a mixture of a titanium tetrahalide, a titanium (IV) alkoxyhalide, and a vanadium oxydihalide with an alkylaluminum compound. It is disclosed further that the molecular weight distribution of polyolefins prepared in the presence of the catalysts can be influenced by adding water or adjusting reaction temperatures during catalyst preparation. A related proposal can be found in U.S. Pat. No. 4,008,358, which discloses olefin polymerization catalysts comprising an alkylaluminum promoter and a solid component prepared by mixing a titanium tetrahalide, a vanadium oxytrichloride, and a vanadium oxyalkoxide and then treating the resulting mixture with an alkylaluminum compound. U.S. Pat. No. 3,678,025 discloses a mixed catalyst system comprising (1) an alkylaluminum reduced titanium or vanadium halide, oxyhalide, alcoholate, alkoxide, or ester, and (2) an alkylaluminum reduced zirconium halide or oxyhalide. It is disclosed that olefins can be polymerized in the presence of such a catalyst system by separately introducing the catalyst components into a polymerization zone, promoting the components with an alkylaluminum compound, and contacting the catalyst with an olefin under polymerizing conditions. It is disclosed further that polymer molecular weight distributions can be controlled by varying the relative concentrations of the reduced titanium and reduced zirconium components.
Despite the above-described prior art proposals, there still is a need for a high activity catalyst useful in the preparation of polyolefins having a broad and controllable molecular weight distribution. It is an object of this invention to provide such a catalyst. A further object of the invention is to provide a catalyst component capable of being varied in terms of olefin polymerization performance by relatively simple means. Other objects of the invention will be apparent to persons of skill in the art from the following description and appended claims.
It has now been found that the objects of this invention can be achieved by reducing a ternary mixture of transition metal compounds, comprising at least one titanium compound, at least one zirconium compound, and at least one vanadium compound, with an organometallic reducing agent and promoting the resulting active catalyst component with an organoaluminum promoter. The activity of the catalyst and the molecular weight distribution of polyolefins produced in the presence thereof are a strong function of the choice of promoter. Moreover, the active component of the invented catalysts can be treated with a deactivating agent to temporarily and reversibly deactivate the component and thereby facilitate the use thereof in polymerization systems in which catalyst is washed into a polymerization zone in an olefin rich recycle stream or otherwise contacts polymerizable monomer prior to entry into a polymerization zone, and further, upon activation of the deactivated catalyst component and polymerization of olefins in the presence thereof, variations in catalyst activities and broadening and narrowing of polyolefin molecular weight distributions can be achieved depending on the choice of deactivating agent. Moreover, the sensitivity of the invented catalysts to hydrogen, commonly used in olefin polymerization to control polymer molecular weights, varies depending on the choice of promoter and deactivator and this allows for simple control of polyolefin melt indices. This promoter, deactivator, and hydrogen sensitivity of the invented catalysts renders the same highly versatile in that the olefin polymerization performance thereof can be varied by relatively simple means such that a single active component can be used to produce polyolefins having a wide range of utilities.
The above-described promoter, deactivator, and hydrogen effects on olefin polymerization performance could not have been predicted on the basis of the performance of known catalysts comprising an organoaluminum promoter and an organoaluminum treated mixture of two transition metal compounds, and it is believed that the combined presence of titanium, zirconium, and vanadium in the active component of the invented catalysts is essential to the olefin polymerization performance thereof. Although known catalysts containing organoaluminum reduced mixtures of titanium and vanadium or titanium and zirconium compounds can exhibit high activities and can be employed in the preparation of polyolefins having a broad molecular weight distribution, such catalysts do not exhibit the promoter, deactivator, and hydrogen sensitivity of the invented catalysts. Attempts to catalyze the polymerization of olefins with organoaluminum reduced mixtures of vanadium and zirconium compounds have met with little success due to poor activities.